A method of the above named kind is described in U.S. Pat. No. 5,251,370 and in the corresponding EP-B-539 793.
The corresponding method is used with a functional element in which the body section or its jacket surface merges via a ring-like contact surface disposed in a plane perpendicular to the longitudinal axis of the functional element into the tubular rivet section, with ribs providing security against rotation being provided in raised form at the contact surface and at the tubular rivet section and preferably having a somewhat right-angled shape. A functional element of this kind is obtainable from the company Profil Verbindungstechnik GmbH & Co. KG under the designation RSN.
The same method can however also be used with a functional element in which the body section or its jacket surface merges via a ring-like contact surface arranged in a plane perpendicular to the longitudinal axis of the functional element into an axial ring-like groove which is bounded at the radially inner side by the tubular rivet section, with the radial groove having a conical wall adjacent to the ring-like contact surface and being bridged by radially extending ribs providing security against rotation. A functional element of this kind is obtainable from the company Profil Verbindungstechnik GmbH & Co. KG under the designation RND. Other elements in which the body section merges via a ring-like contact surface disposed in a plane to the longitudinal axis of the functional element directly or indirectly into the rivet section can be secured to a sheet metal part with the named method.
Whereas the RSN element is shown in the said EP-B-539 793 the RND element is claimed in the European patent application 01 109 757.3.
Both elements, i.e. the both RSN element and also the RND element will be secured to a sheet metal part using the so-called clamping hole riveting method in accordance with EP 539 793 B1.
This method is carried out in such a way that a hole extending through the sheet metal part is formed, in that the sheet metal part is drawn and plastically deformed into a generally dome-like section which surrounds the hole, with the hole being disposed at a smaller diameter of the dome-like section, the tubular section of the functional element is inserted through the hole into the sheet metal part and the dome-like section of the panel is plastically deformed, whereby the diameter of the hole is reduced and the surrounding material is brought into engagement with the tubular rivet section. At the same time as the pressing flat of the dome-like section of the sheet metal part into a generally planar shape an end of the tubular rivet section is simultaneously plastically deformed radially outwardly, whereby a mechanical interlock is formed between the sheet metal part and the functional element. In other words, the diameter of the hole in the sheet metal part is reduced and at the same time the tubular rivet section with the functional element is dilated radially outwardly as a result of the action of the riveting die button, so that a firm pressure of the sheet metal part against the tubular rivet section takes place.
The hole is dimensioned so that, if the sheet metal part is pressed flat without an element inserted therein, the inner diameter of the hole is smaller than the outer diameter of the tubular rivet section. This dimensioning of the hole ensures that the desired “strangling action” between the sheet metal part and the tubular rivet section takes place. In this way a permanent compressive stress is to be produced both in the sheet metal material around the edge of the hole and also in the tubular rivet section and in the installed state. This compressive stress leads to a high contact force, and thus friction, at the hole between the sheet metal part and the tubular rivet section, whereby a considerable security against rotation is achieved between the functional element and the sheet metal part independently of the noses providing security against rotation. Furthermore, this permanent compressive stress provides a security against fatigue cracks because these can only arise if the compressive stress changes as a result of loading into a tensile stress. As a result of the compressive stress such tensile stresses and thus fatigue cracks can be effectively prevented.
It is not necessary for the sheet metal part to be first deformed into a dome-like section and then to produce the hole in the dome-like section but rather the hole can be produced first and then the sheet metal part can be deformed into a dome-like section, or the formation of the dome-like section and of the hole can take place simultaneously.
It can be seen from the above quoted description of the known method that the dome-like section is essential in order to ensure that on pressing flat or partial pressing flat of the dome-like section a reduction in size of the hole is achieved. Furthermore it can be seen from the description that the hole is to have a diameter which is only fractionally larger than the diameter of the tubular section. In practice the hole in the dome-like section has a diameter prior to the attachment of the functional element which exceeds the diameter of the tubular section by about 0.3 mm.